Biometric authentication is a technique for identity verification using biometric information such as a fingerprint, face, vein, or the like. Biometric information that is registered in advance, and biometric information that is acquired from a person when confirmation is required are compared (or collated), and identity of this person is verified when the compared biometric information match. For example, biometric authentication using a palm-print pattern, a palm shape (or lines of the palm), a palm vein pattern, or the like is employed in terminals including laptop PCs (Personal Computers), tablets, smartphones, or the like.
In the biometric authentication apparatus that uses an image of the palm (hereinafter also referred to as “a palm image”) such as the palm-print pattern, the palm shape, and the palm vein pattern, or the like, as the biometric information, an area that is captured and an image that is captured greatly differ between a case in which the palm is captured in an open (or flat) state and a case in which the palm is captured in a closed (or rounded) state. When the open or closed state of the palm differs between a time when the palm is registered and a time when the palm is captured for identity verification, an authentication accuracy deteriorates. Consequently, a false rejection rate may increase due to erroneous authentication. For this reason, it is desirable to judge whether the palm is open or closed, when registering the palm and when capturing the palm for identity verification. Various techniques have been proposed to judge the open or closed state of the palm.
According to a first example, the open or closed state of the palm is judged from a ratio of a vertical length and a horizontal length of the image of the entire hand that is captured. This first example is proposed in International Publication Pamphlet No. WO2004/021884, for example. In addition, a second example utilizes for the authentication, gesture images that are obtained by changing the open or closed state of the palm. This second example is proposed in Japanese Laid-Open Publication No. 2008-90412, for example. According to this second example, the entire hand is captured at each gesture state when registering the image of the hand, and the image of the hand that is captured when making the authentication is matched against the registered images of the hand, to judge the gesture state of the captured image of the hand.
The biometric authentication apparatus that uses the palm image efficiently captures the palm with a high picture quality. For this reason, as described in David D. Zhang, “Palmprint Authentication”, Springer, Chapter 6, pp. 73-83, 2004, for example, popularly employed systems utilize for the authentication the palm image that is captured in the state in which the palm is open.
However, in the case of the first example, it is a precondition that the entire hand is captured. Hence, it is impossible to obtain the ratio between the vertical length and the horizontal length of the image of the entire hand, from the palm image corresponding to a part of the entire hand. For this reason, according to the first example, it is difficult to accurate judge the open or closed state of the palm from the palm image corresponding to a part of the entire hand. On the other hand, in the case of the second example, if a number of kinds of gesture images that are registered in advance is limited, it is difficult to accurately judge the open or closed state of the palm by matching the gesture images.
Accordingly, it is conventionally difficult to accurately judge the biometric state.
Other examples of related art include International Publication Pamphlet No. WO2012/111664, International Publication Pamphlet No. WO2013/005305, Japanese Laid-Open Patent Publication No. 2006-107288, David D. Zhang, “Palmprint Authentication”, Springer, Chapter 6, pp. 73-83, 2004, Akihiro Shimada et al., “Analysis of Friction Characteristics on Human Fingers”, Transactions of the Society of Instrument and Control Engineers, Vol. 32, No. 12, pp. 1581-1587, 1996, and Takashi Maeno et al., “Analysis on Geometry of Human Epidermal Ridges”, Transactions of the Japan Society of Mechanical Engineers (Series C), Vol. 71, No. 701, pp. 245-250, 2005, for example.